In series circuit arrangements of power semiconductors, for instance in so-called high-voltage direct-current transmission converters (HVDC), a cooling circuit is employed. The local potential distribution of a cooling circuit is adapted by control electrodes to the voltage of the electrical components in the immediate surroundings. This is done to prevent partial discharges between the cooling-water-carrying pipelines made of synthetic material and the electrical components. Furthermore, corrosion currents that are brought about by differences of potential between the cooling water and the metallic system components coming into contact with the cooling water are kept down in this manner. Since the control electrodes become coated in the course of time with mechanically hard coverings which can chip off and clog up cooling lines, the control electrodes have to be cleaned or exchanged in the course of the periodic shutdown of the system.
Conventional series circuit arrangements exhibit cooling-water distributor lines, wherein, starting from a cooling-water inlet or cooling-water outlet, submodules of series circuits are arranged on both sides and a region around the cooling-water inlet is, for structural reasons, as a rule not equipped with corresponding power-semiconductor devices and cooling chambers. For the purpose of controlling the potential in each of these submodules, two control electrodes are ordinarily provided respectively in each submodule on both sides of the cooling-water inlet and cooling-water outflow, said electrodes being connected to the first cooling chambers on both sides, so that the potential in the interspace—that is to say, in the region of the inlet—is kept at the mean value of the potentials at the ends of the distributor. Consequently, in a conventional series circuit arrangement four control electrodes are provided in each instance per cooling-water distributor line—that is to say, with reference to two cooling-water distributor lines in each instance, a total of eight control electrodes.